The present specification relates generally to the field of integrated circuits and to methods of manufacturing integrated circuits. More particularly, the present specification relates to a dark field trench in an alternating phase shift mask to avoid phase conflict.
Semiconductor devices or integrated circuits (ICs) can include millions of devices, such as, transistors. Ultra-large scale integrated (ULSI) circuits can include complementary metal oxide semiconductor (CMOS) field effect transistors (FET). Despite the ability of conventional systems and processes to put millions of devices on an IC, there is still a need to decrease the size of IC device features, and, thus, increase the number of devices on an IC.
One limitation to the smallness of IC critical dimensions is conventional lithography. In general, projection lithography refers to processes for pattern transfer between various media. According to conventional projection lithography, a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film or coating, the photoresist. An exposing source of radiation (such as light, x-rays, or an electron beam) illuminates selected areas of the surface through an intervening master template, the mask, for a particular pattern. The lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive image of the subject pattern.
Exposure of the coating through a photomask or reticle causes the image area to become selectively crosslinked and consequently either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble (i.e., uncrosslinked) or deprotected areas are removed in the developing process to leave the pattern image in the coating as less soluble polymer.
Projection lithography is a powerful and essential tool for microelectronics processing. As feature sizes are driven smaller and smaller, optical systems are approaching their limits caused by the wavelengths of the optical radiation.
Conventional projection lithographic processes are limited in their ability to print small features, such as, contacts, trenches, polysilicon lines or gate structures. As such, the critical dimensions of IC device features, and, thus, IC devices, are limited in how small they can be.
The ability to reduce the size of structures, such as, shorter IC gate lengths depends, in part, on the wavelength of light used to expose the photoresist. In conventional fabrication processes, optical devices expose the photoresist using light having a wavelength of 248 nm (nanometers), but conventional processes have also used the 193 nm wavelength. Further, next generation lithographic technologies may progress toward a radiation having a wavelength of 157 nm and even shorter wavelengths, such as those used in EUV lithography (e.g., 13 nm).
Phase-shifting mask technology has been used to improve the resolution and depth of focus of the photolithographic process. Phase-shifting mask technology refers to a photolithographic mask which selectively alters the phase of the light passing through certain areas of the mask in order to take advantage of destructive interference to improve resolution and depth of focus. For example, in a simple case, each aperture in the phase-shifting mask transmits light 180 degrees out of phase from light passing through adjacent apertures. This 180 degree phase difference causes any light overlapping from two adjacent apertures to interfere destructively, thereby reducing any exposure in the center xe2x80x9cdarkxe2x80x9d comprising an opaque material, such as chrome.
An exemplary phase-shifting mask 10 is illustrated in FIG. 1. Phase-shifting mask 10 includes a transparent layer 12 and an opaque layer 14. Opaque layer 14 provides a printed circuit pattern to selectively block the transmission of light from transparent layer 12 to a layer of resist on a semiconductor wafer. Transparent layer 12 includes trenches 16 which are etched a predetermined depth into transparent layer 12. The light transmitted through transparent layer 12 at trenches 16 is phase-shifted 180 degrees from the transmission of light through other portions of phase-shifting mask, such as portions 18. As the light travels between phase-shifting mask 10 and the resist layer of a semiconductor wafer below (not shown), the light scattered from phase-shifting mask 10 at trenches 16 interferes constructively with the light transmitted through phase-shifting mask 10 at portions 18, to provide improved resolution and depth of focus.
As mentioned, various different wavelengths of light are used in different photolithographic processes. The optimal wavelength of light is based on many factors, such as the composition of the resist, the desired critical dimension (CD) of the integrated circuit, etc. Often, the optimal wavelength of light must be determined by performing a lithography test with photolithographic equipment having different wavelengths. When a phase-shifting mask technique is utilized, two different phase-shifting masks must be fabricated, each mask having trenches 16 suitable for phase-shifting light of the desired wavelength. The fabrication of phase-shifting masks is costly. Further, comparison of the effect of the two different wavelengths printing processes is difficult and requires complex software processing to provide a suitable display.
One difficulty in using phase-shifting mask technologies is phase conflict. Phase conflict arises when two separate areas on a phase-shifting mask have the same phase shift characteristic and are so close in proximity that there is a bridging between the two areas. Bridging, or the effective photo-connection of two separate areas in the mask, results in a less than accurate mask. As such, phase-shifting masks are designed to avoid proximity of areas where the light will have the same phase going through both areas. This design constraint can limit the size and complexity of the phase-shifting mask, and, thus, the pattern on the IC.
Thus, there is a need for an improved phase-shifting mask. Further, there is a need for avoiding phase conflict issues in phase shift masks. Further still, there is a need for a dark field trench in an alternating phase shift mask having a high transmittance area to avoid phase conflict.
An exemplary embodiment relates to a photoresist mask used in the fabrication of an integrated circuit. This mask can include a first portion having a phase characteristic, a second portion being located proximate the first portion and having the same phase characteristic as the first portion, and a segment disposed between the first portion and the second portion to prevent phase conflict between the first portion and the second portion.
Another exemplary embodiment relates to a photoresist mask configured for use in an integrated circuit fabrication process. This mask can be made by a method including depositing a phase shift material over an opaque layer, and selectively removing the phase shift material except at a location between two phase shift mask portions having the same phase characteristic.
Another exemplary embodiment relates to a phase shifting mask. This phase shifting mask can include a first section with an alternating phase shift characteristic, a second section which is proximate to the first section and has the same alternating phase shift characteristic as the first section, and a third section with a high transmittance attenuating phase shift characteristic being formed at the location of the potential phase conflict section. A potential phase conflict section is located between the first section and the second section.
Other principle features and advantages of the present invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims.